This application claims priority under 35 U.S.C. xc2xa7119 from Canadian Patent Application No. 2,292,170 filed on Dec. 14, 1999.
The present invention relates to a metal anchor joint for anchoring casing in a well and to tee process of making and using it. More particularly the anchor joint is a thick-walled steel tubular, such as a length of well casing, having outwardly protruding rings affixed thereto.
Well structures installed in the earth to exploit geothermal or petroleum energy resources are typically lines with tubular steel casings, which in turn, are cemented in place within the well bore. Under certain conditions, such as significant temperature changes, the casing tends to displace axially relative to the adjacent earth material. The present invention provides a means to restrain such relative displacement.
Within the context of petroleum drilling and completion systems, the vast majority of casing systems need only accommodate the loads arising from installation prior to cementing, and non-thermal production methods after cementing. For these conventional production methods, casing designs typically only consider pressure containment, collapse resistance and hydraulic isolation requirements, and not axial load changes after cementing.
However, in thermal applications, or where ground movements induced by processes such as reservoir compaction may occur, it is often desirable to provide highly efficient axial load transfer over relatively short interval lengths to prevent casing movement and consequent damaging effects on adjoining or attached components of the completion system.
The present invention was conceived specifically as a means to restrain the axial movement of casing strings in well bores which will be used for production of heavy oil by means of the process of steam stimulation. When casing is heated, axial displacement resulting from thermal expansion tends to occur and be concentrated at locations coincident with changes in the axial strength of the tubulars.
These axial displacements are most obvious at ground surface, where the casing ends. Movement at this location typically cases the well head to rise and fall relative to ground surface, correlative with increases and decreases in temperature, respectively. Surface piping connected to the well head must therefore include provisions to accommodate this movement or risk failure. Such provisions and risk increase cost; therefore a cost effective and reliable means to reduce surface well head displacement by restraining or anchoring the casing is advantageous.
Less obviously, changes in axial strength may occur down hole at locations where there is a transition in size, grade or configuration of components in the casing string. For example, such changes occur at liner junctions, or where axially compliant devices such as corrugated tubulars are employed. At these locations, axial movement of the casing occurs relative to the adjacent formation; this tends to concentrate strain in the weakest member of the string, potentially causing it to fail with consequent loss of either structural or pressure integrity.
Because of the generally long string lengths employed to case wells, the magnitude of axial load transferred between the casing and surrounding earth materials through the cement sheath is usually very low, and for typical non-thermal applications, is largely static. Therefore, there has apparently been little interest in developing methods to improve the efficiency of axial load transfer between the casing and cement sheath, beyond what occurs xe2x80x98naturallyxe2x80x99 by friction and interlocking at the upset surfaces at connection points.
Even where axial load transfer is considered, the conventional understanding of interaction between the pipe and cement as described by D. K. Smith in xe2x80x9cCementing,xe2x80x9d SPE Monograph Vol. 4, Society of Petroleum Engineers Inc. January, 1990, anticipates that a cement bond exists, capable of transmitting shear between the casing and cement and hence transferring axial load. This reference reports measured xe2x80x98bondxe2x80x99 strengths ranging from 20 to over 200 psi. These values were derived from cemented tube-in-tube tests where the annular space between two lengths of pipe was cemented. Axial compressive load was then applied to one tube and reacted by the other. For these tests, the effective (radial) stress present across the cement to steel tubular interface is not reported or considered, and the total reported average xe2x80x98bondxe2x80x99 strength is considered adhesive. Hence, designs that do consider axial load transfer typically rely on the presence of this apparent bond mechanism that, if present, would provide substantial load transfer over a relatively short axial length. For example, given a bond strength of 100 psi (which is about mid range of the values reported) a 7 inch diameter pipe could develop a calculated axial load resistance of 500,000 lb over just 18.95 feet. However, as described by Schwall, G. H., Slack, M. W. and Kaiser, T. M. V. in xe2x80x9cReservoir Compaction Well Design for the Ekofisk Fieldxe2x80x9d. SPE Paper 36821, 1996 SPE Annual Technical Conference and Exhibition, Denver, Oct. 6-9, 1996, the concept of significant adhesive cement bond was alleged to be erroneous. The interaction behavior between the cement and steel was explained as a frictional mechanism.
While significant frictional forces may be developed along the casing length at depth, this may not always be relied upon, particularly at shallow depths.
With this background in mind, it is the objective of the present invention to provide anchoring means, for incorporation in a casing string, which is intended to function to reduce relative movement between the string and the adjacent earth material.
In accordance with the invention, an anchor joint for incorporation in a casing string is provided. The anchor joint comprises a thick-walled metal tubular having means (e.g. threads) at its ends for connection with the casing string. The tubular has a plurality of outwardly projecting, abrupt diameter changes spaced along its length.
More particularly, one or more metal rings are crimped or shrink fitted onto the tubular. Preferably, in its instressed condition (that is, prior to crimping or shrink fitting), each ring has an inner diameter equal to or less than the original outside diameter of the tubular.
In a more preferred embodiment, at least one steel ring is crimped onto a steel joint of well casing. The ring has a yield strength less than that of the joint. Crimping may be carried out by hydroforming. As a result of crimping both the joint wall and the ring, a detent is formed in the joint side wall and the ring is trapped within the detent.
By locking the joint and rings together by crimping or shrink fitting, the resulting engagement is sufficient to enable the joint to transfer axial load from the casing string through the ring to the surrounding cement sheath of the well, to provide resistance to axial displacement of the anchor joint relative to the earth material.
As stated, the tubular is xe2x80x9cthick-walledxe2x80x9d. In a general sense, this word is intended to convey that the anchor joint tubular wall is sufficiently strong and thick so as to maintain the structural integrity of the casing string. More specifically, it means that the tubular has a diameter to thickness ratio (xe2x80x9cD/txe2x80x9d) less than 100, preferably less than 50. Most preferably the tubular is a joint of the casing used in the casing string. By being thick-walled and having end connections, the tubular is compatible with the casing string.
By xe2x80x9cabruptxe2x80x9d is meant that the diameter changes create shoulders that preferably are substantially perpendicular to the axis of the tubular or alternatively may be sloped with an angle of at least 20xe2x80x3, more preferably at least 45xc2x0, relative to the axis of the tubular.
Preferably the joint will have a length in the order of 40 feet, so that it conforms with the average length of casing joints.
It will be apparent that the ability to efficiently transfer axial load between the anchor joint and the wellbore wall through the confining material such as cement typically placed in the annulus between the anchor joint and wellbore wall will depend on the tendency of the multiple abrupt diameter changes to displace the confining material as axial movement is attempted. To provide a significant improvement in the anchoring function of a threaded and coupled anchor joint, the total volume swept by the multiple abrupt diameter changes preferably should be of the same order as that already swept by the face of the joint coupling or collar for a given amount of axial movement. This collar face area is typically approximately equal to the joint body cross-sectional so that the swept volume is this area times the axial displacement. Therefore it is preferred that the relevant upper or lower shoulder areas of the diameter changes of the anchor joint shoulds in total create an area equal to the cross-sectional area of the anchor joint body. Otherwise stated, the total axial area presented by the diameter change or shoulder to the confining material in the direction of movement should preferably be at least equal to the cross-sectional area of the anchor joint tubular body.
In addition, the diameter changes preferably should be of sufficient magnitude to result in significant inter-penetration with the confining material. There may be gaps between the confining material and the anchor joint tubular outer surface, such as the micro-annulus reported to occur between cement and a tubular. In addition, the radial stiffness of the confining material may allow it to deflect away from surfaces where the diameter change tends to cause loading during axial displacement of the casing string. For these reasons, it is preferred that the diameter changes be greater than 0.5% of the tubular diameter, more preferably greater than 1% of the diameter.
In a preferred embodiment, the anchor joint comprises a joint of steel well casing having external, solid steel rings affixed, as by crimping, in locking engagement with the tubular wall.
Preferably the rings are cylindrical, have a thickness abut equal to the tube wall thickness and are spaced apart at least 10 ring thicknesses.
The number of rings and the length of the anchor joint should be selected with a view to providing adequate shoulder contact with the cement or other confining material to react the axial load tending to cause movement of the casing. Selecting the number of rings, the length of anchor joint and the frequency of anchor joints will in part be determined by field experience.
In another preferred embodiment, the invention is concerned with a method for anchoring a casing string in a wellbore comprising: inserting a plurality of anchor joints at spaced intervals into a casing string as the string is being run into the wellbore; each anchor joint comprising a joint of casing having a plurality of external steel rings affixed by crimping or shrink fitting in locking engagement with the joint at spaced positions along the joint, and cementing the anchor joints in the wellbore.
Each crimp ring is preferably secured to the tubular by a hydroforming process comprising:
(a) providing a thick-walled metal tubular compatible with a casing string;
(b) positioning a crimp ring around the tubular, the ring being formed from a ductile material, such as steel, having a yield strength less than the tubular, the ring having an internal diameter slightly greater than the external diameter of the tubular and an external profile comprising end sections and a middle section of reduced outside diameter relative to the end sections;
(c) providing a pressure forming vessel around the ring, the vessel having an internal bore slightly larger than the outside diameter of the ring;
(d) the forming vessel having internal grooves, carrying seals, spaced to straddle the reduced diameter ring middle section and to seal against the end sections to define a pressure chamber between the seals;
(e) providing a stop tube having a length at least equal to that of the ring, within the tubular in opposed relation to the ring, the stop tube preferably having an outside diameter less than the inside diameter of the tubular by an amount at least equal to twice the elastic limit displacement of the tubular;
(f) the vessel having a passage extending through its wall to communicate with the pressure chamber;
(g) introducing pressurized liquid into the pressure chamber through the passage and causing the ring and tubular side wall to deform inwardly until the side wall contacts the stop tube and the ring is affixed to the tubular; and
(h) repeating the foregoing steps to affix a plurality of rings to the tubular to produce an anchor joint.
As a further step, the anchor joint so produced is connected into a casing string and introduced into a wellbore and cemented in place.